1. Field of the Invention
This invention relates to the preparation of 8-hydroxyquinolines and their use as metal extractants.
2. Statement of the Related Art
Substituted and non-substituted 8-hydroxyquinoline compounds are well-known for their ability to coordinate with ions of a variety of transition metals to form relatively stable complexes commonly referred to as metal chelates. This complexing technique has been used extensively in chemical analytical procedures. Chelates have also been utilized in hydrometallurgical extraction processes applicable for the recovery of metal values from aqueous solutions thereof. These extraction processes comprise basically a two step operation. In the first step an impure aqueous phase containing the desired metal values in ionic form is contacted with a water-immiscible organic solution of the extractant whereby the metal ions are extracted into the organic phase to form a chelate with the organic extractant. The second step, referred to as stripping, transfers the extracted metal values into an aqueous phase regenerating the metal ions, and forming a relatively concentrated solution of the desired metal. The metal can then can then be recovered from this solution by any convenient recovery method, such as electrolysis.
Substituted 8-hydroxyquinolines compounds having bulky hydrocarbon substituents are used to enhance the hydrocarbon solubility characteristics of the 8-hydroxyquinoline. Substituents selected on the basis of their carbon atom content will reduce the extractant's solubility in strongly acidic aqueous mediums and increase hydrocarbon solubility. The molecular configuration of the substituent, moreover, can be used to increase the loading capabilities of the base compound.
One example of such substituted 8-hydroxyquinoline extractant is described in U.S. Pat. No. 3,637,711. This patent describes substituted 8-hydroxyquinolines characterized by having a beta-alkenyl group in the number 7 position. A similar substituted 8-hydroxyquinoline also used in metal extraction is described in U.S. Pat. No. 4,066,652, describing a class of alpha-alkenyl substituted 8-hydroxyquinolines.
One disadvantage of these extractants is encountered with hydroxyquinolines having alpha- or beta- unsaturated hydrocarbon substituents in the 7 position. This unsaturation, moreover, is difficult or impossible to remove, even by hydrogenation. These alkenyl substituted 8-hydroxyquinolines have the disadvantage of instability, especially when contacted with strong alkali and air at elevated temperatures. Such conditions cause the double bond in the side chain to slowly react with the phenolic oxygen in the 8 position to form a cyclic ether having little or no activity as a solvent extraction reagent. An extractant without the alpha- or beta- unsaturation on the 7-position substituent, therefore, would have significant advantages in metal recovery, especially if this extractant also has an enhanced hydrocarbon solubility.
The instant invention describes substituted 8-hydroxyquinoline derivatives having a hydrocarbon substituent in the 7-position which does not have alpha or beta unsaturation. It is also an object of the instant invention to provide alkylated 8-hydroxyquinoline derivatives which can have increased organic solubility by the addition of further saturated or unsaturated hydrocarbon moieties to the compound. The instant invention also provides substituted 8-hydroxyquinoline metal extractants which can have a variety of substituted or unsubstituted cyclic hydrocarbon sturctures. This provides a useful tool for varying activity, solvents, and other extractant characteristics. Other objects will become apparent as this description proceeds.